Question

Why are cells are so small in size?

Answer 1

Small cells are more efficient than larger cells.If the cells are small they will have more surface area(cell membrane ) than cytoplasm volume.More surface area of the cell will help in absorbing materials easily.If the cell is big, volume incrases much more than the incrase in surface area.So the cell cant take whatever materials it wants efficiently.
So cell cant function properly. That is why the cells have to be smaller in size.